Drill pipes often get stuck in the hole during the drilling process carried out oil and gas drilling operations. The main reasons of this undesirable situation are as follows:                insufficient drilling mud circulation, which results in accumulation of sludge in the hole;        insufficient drilling mud weight, which results in caving;        excess drilling mud weight, which results in sticking;        peculiarities of rock lithology (e.g. water-sensitive clays which swell in the presence of water);        peculiarities of rock structure (e.g. some sedimentary rock may form long narrow lenses);        tangential tectonic stresses, which results in caving; improper drilling mud composition, which results in inefficient or easily peelable mud cake;        various faults of the drilling rig, derrick and underwater equipment, which results in long interruptions of pipe rotation, drilling mud movement or circulation;        various faults of the pipe string; and        human factors.        
Pipe recovery is always unwanted but is frequently a necessary operation in the drilling. If a tubular (such as drillpipe, heavy weight drillpipe, drill collar, stabilizer joint, production tubing, casing, or liner) becomes stuck in the borehole and cannot be retrieved by activating downhole jar devices, applying pull or torque from surface, or adjusting mud circulation, the usual practice is to disconnect the free part above the stuck point (by means of various mechanical, explosive or chemical devices), and retrieve the free portion of the tubular string from the well. Upon the retrieval, remedial actions can be applied to the remaining portion of the string.
Since pipe sticking contributes to down time during drilling, it is important to resolve this problem quickly. Pipes get stuck in almost a third of drill holes at the preliminary drilling (so-called exploration drilling) stage.
If standard stuck-pipe releasing measures such as activation of drilling jars, increased drilling mud circulation, changes in the drilling mud weight, etc. prove to be inefficient, a remedial pipe recovery procedure is started. The typical pipe recovery sequence is as follows:                1. Determination of the most likely location of the “free point” 1, i.e. the lowest pipe string section which is still free; see FIG. 1. FIG. 1 shows a stuck pipe 2 in a borehole 3 below the derrick floor level 4 with a wireline tool 5 in the pipe 2.        2. Resumption of drilling mud circulation: in some cases, it is recommended that the pipe below the free point should be perforated and that the drilling mud circulation should be resumed from this point upwards. A strong drilling mud flow can displace the obstacle upwards.        3. Pulling of free pipe: the pipe above the free point is separated from the stuck bottom part and can be pulled out to the surface. Many sophisticated mechanical, explosive and chemical aids are used for separation of the pipe string.        4. After the free pipe has been pulled out, fishing operations are commenced to retrieve the remaining part of the pipe string and to pull it from the hole; see FIG. 2 for various typical rig-up tools used. In case of a reliable fish, the sequence returns to step 1. described above, but the drilling connection now includes additional drilling jars, a fishing slip to be used for fishing the remaining part, and a safety joint for quick disconnection in case of further troubles.        5. If the fishing operations are successful, the drilling process continues as usual. If the fishing operations fail, the driller will have an option either to drill a side hole to bypass the remaining part of the string or to abandon the whole borehole. It is important to understand that, without performing the pipe pulling operation (according to 2 above), it is impossible to eliminate the emergency by bypassing the remaining part of the string via the second hole or to abandon the borehole in a safe and environmentally appropriate way.        
It is important to have a good estimation of the bottom-most free point in the tubular string. Performing retrieval from too far above this free point results in loss of useful borehole space and in unnecessary loss of expensive tubulars. Cutting the tubular string below the free point obviously results in no retrieval at all and severely complicates any possible remedial actions.
As shown above, the free point detection procedure is important for successful accomplishment of the pipe-pulling operation and can even be used several times during the same attempt to pull the pipe. Emergency pulling of a string part is one of the most dangerous operations on the derrick and has the potential to cause injuries and even death of the personnel.
Presently, there are three conventional methods of determination of the free point.
1 Determination of the Free Point, Based on Measurements of Pipe Extension from the Surface.
First, the buoyancy of the drill pipe should be determined. This buoyant force can be calculated, using special tables based on the specific gravity of the drilling mud, type and length of the drilling pipe. The calculations are checked, using a weight indicator on the hook suspending the drill pipe, by comparing the calculated buoyant force with average hook readings, while moving the pipe up and down until equilibrium has been determined (the averaging of these measurements reduces the impact of errors on friction).
After the pipe has been placed in equilibrium, a chalk mark is made on the drill string at the derrick floor level. The driller slowly applies a tension force exceeding the buoyant force, i.e. by a specified value greater than the buoyant force, and the driller's assistant measures and records the pipe extension (i.e. the position of the chalk mark above the derrick floor level).
The stuck point is assessed, based on the linear pipe extension/tension force relationship. The shorter the pipe extension for a fixed drag force, the shallower the depth at which the free point is located. The drillers are accustomed to perform the tubular stretch measurements from the surface by applying different values of over-pull to the stuck tubular and performing stretch measurements of the tubular at the rig floor.
Tables of pipe extension coefficients and nomograms to be used for determination of the free point are published for most pipes. Recently, special software has been developed for laptops and palmtops to allow the performance of the same calculations even in cases that the drill string consists of different pipes.
The overall accuracy of this method is limited by the resolution of the weight indicators on the hook and by the general design of the traveling block and draw-works drums of the drilling rig. The measurements are also influenced by the friction between the drill pipes and the hole walls in deviated holes. Thus, surface determination of the stuck point is always performed but is almost always supplemented with and confirmed by other types of measurements which are described below.
Also, if the well is deviated and/or the stuck point is close to surface, such measurements become difficult, imprecise, or impossible.
2 Downhole Determination of the Free Point Based on Attachment of Stress and Torque Sensors.
Another conventional method is to use precise electromechanical stretch and torque sensors that can be attached to the inside of the tubular by means of remotely operated anchors. Pipe stretch and torque can be recorded, point-by-point, by such sondes whilst the stretch and torque is applied from the surface by the driller. If the sensor indicates any movement (stretch or torque), then the anchoring point is above the free point. If the sensor does not indicate any movement, then the anchoring point is below the free point.
Fixed stress and torque sensors have been used for development of cable measurement methods since the early 1960s. The latest example of such tools is a Free Point Indicator Tool™ (FPIT) developed by Schlumberger. The tool can be installed on a conventional 7-conductor logging cable.
The tool consists of two independent electromechanical anchor sections spaced 2 meters apart, and of a stress and torque precision sensor installed between them. Anchor motors can be enabled from the electronic module installed above the upper anchor. The same electronic module digitizes the sensor signals and sends them to the surface into a computer-aided measurement results management and gathering system.
FIG. 2 shows a typical drill floor setup for a wireline tool run into a stuck drill pipe. The drill pipe 20 is supported on the derrick (not shown) by means of a hook 22 and draw works including a running block 24. The wireline tool (not shown) is run inside the drill pipe 20 on a wireline cable 26 via an upper block 28 and a sheave (and cable odometer) 30. Measurements start from determination of equilibrium, as described above. Logging cable blocks are located on the derrick: the lower one is installed into standard position at the bottom and the upper one is fixed on the derrick structures. The upper block cannot be placed into standard position on the travelling block because this block is also used for application of a tension force to the pipes. The tool is then lowered into the stuck pipe string.
The driller applies a force equal to the buoyant force. The upper anchor is activated at a certain predetermined point at the command from the surface, and the tool is fixed on the pipe. Then, the cable tension is slackened so that accidental cable movement should not influence the measurement results. After that, the lower motor is activated.
First, it resets the sensor block by setting it into the slack and untwisted initial condition and then extends the lower anchor. After that, the driller slowly applies a tension force exceeding the buoyant force by a specified value, and the operator of the logging system reads the sensor. If the pipe is free at the anchor fixation point, the sensor registers axial movement of the upper anchor with respect to the lower anchor. Depending on the derrick design, the driller can then apply a torque to the drill pipe in specified increments with respect to the normal position, and the operator reads the sensor.
If the pipe is free at the point where the anchor is located, the sensor registers a turn of the upper anchor with respect to the lower anchor. After the measurement has been taken, the cable slack is taken up, the anchors (first the lower one, and then the upper one) fold up, and the tool can be moved to the next measurement point where the whole procedure is repeated.
Using this method, it is possible to determine the free point to a required degree of accuracy after 10-15 measurements. Limitations of this method are connected with the physics of measurements. The sensor must be very sensitive and must register weak relative movements of the anchors. So, the measurements are influenced by the cable friction inside the pipes and by the cable position on the derrick (especially if the cable is in contact with a part of the moving block 24). The measurements can further be influenced by anchor slips. If the inside diameter of the pipe exceeds 80 mm, the reliability of the measurements will be reduced due to the curvature of the anchor legs. The necessity of continuous pipe movement endangers the personnel on the derrick and measurements are taken very slowly. The measurements taken using a FPIT are considered to be “sensitive to the personnel qualification” and require the availability of an experienced logging operator.
3 Downhole Determination of the Free Point, Based on Magnetic Marks.
The third conventional method of free point estimation by wireline tool is to record magnetic marks from inside the tubular downhole and then apply stretch from surface. The position and the strength of the magnetic marks can be recorded. In the section of the tubular below the free point, both strength and the position of the marks remain unchanged, while in the portion above the free point, changes are observed.
The method of magnetic marks (Russian Inventor's Certificate 142242 E 21 B 23/09, 1961) is often used by field logging companies which developed from former USSR/CIS′ enterprises, and this method has been known since the early 1960s.
The tool depicted in FIG. 3 consists of a diamagnetic shell 6 with a paramagnetic core 7 in the form of a coil. Electric winding 8 is wound on the coil in such a way as to form an open-core electromagnet. The sensitive part of this tool is manufactured in different diameters and, consequently, the slot between the pipe wall and the magnetic core is limited.
Measurements start from determination of equilibrium, as described above. The logging cable blocks are installed on the derrick: the lower one is installed into standard position at the bottom and the upper one is fixed on the derrick structures. According to another option, the upper block is placed into standard position on the traveling block. In this case, the tool can be temporarily pulled from the pipes as long as the traveling block is used for application of a tension force which is then maintained by using borehole wedges. Depending on the derrick design, this option can be much safer and faster as compared with the option in which the upper block is located on the stationary structure of the derrick.
The driller applies a force equal to the buoyant force. The logging tool is lowered to the bottom of the pipe to make the “marking pass”. At a preliminary selected distance (the achievement of this distance is determined, using a cable odometer), heavy current is supplied to the coil, which results in magnetization of a narrow ring of the drill pipe wall. After that, the tool is lowered once more to make the “base pass”. The coil is connected to the sensitive electronic block that measures electric tension in the coil and determines magnetization along the length of the pipe walls. Then, the coil is again lowered to the bottom, and the driller applies a drag force from the surface. The tool makes the “stretched pass” and records the level of magnetization of the pipe walls.
The data obtained from the “base pass” and the “stretched pass” are compared to draw a conclusion about the free point. The position and the intensity of magnetic marks will remain unchanged in the area below the free point. As far as the area above the free point is concerned, the distance between the magnetic marks will slightly increase and their intensity will decrease.
Limitations of this method are connected with the fact that the drill pipe must only be made of steel having a sufficient coercive force so that the pipe could retain magnetization. This method is not applicable to paramagnetic strings made of aluminum, stainless steel or Monel, for instance. The applicability of the method is adversely affected by the fact that the position of the mark is associated with the logging odometer readings, and the accuracy of determination of the distance between the magnetic marks is therefore inevitably limited by depth measurement errors and is connected with a well-known mathematical problem of “small difference of big numbers”.
There is a known method (Russian Inventor's Certificate 600287 E 21 B 23/00, 1978) of determination of the stuck point of a drill pipe string. According to the known method, when determining the stuck point, drillers lower a stuck point detector, using a logging cable, into the stuck pipe string to reach the stuck point, and make a control record of changes in magnetic properties along the pipe string within the assumed stuck point range in the selected depth scale. The stuck point detector used during the implementation of the method contains a power point, a tool head, a non-magnetic protective shell and a cored coil, as well as a condenser, a diode and a gas-discharge lamp located in an insulating sleeve. The gas-discharge lamp is placed between the power point and the coil in parallel with the diode, and the condenser is placed in parallel with the coil and the gas-discharge lamp.
The disadvantage of the known method consists in the fact that the results of the stuck point determination greatly depend on the previous magnetization of the pipe and that it is impossible to use this method in paramagnetic strings.
There is also a known method (Russian Inventor's Certificate 1420148 E 21 B 47/09, 1988) of determination of the boundary of the stuck area of a drill pipe string in a hole. According to the known method, a stationary magnetic field corresponding to the maximum differential permeability of the string material is created in the specified area of the drill pipe. While the string is gradually and mechanically loaded, a Barkhausen effect occurs in the free area and is registered. The Barkhausen effect consists in occurrence of pulse electric current or tension in the chain of the inductance coil located near the surface of the ferromagnetic object. The boundary of the stuck area is determined by disappearance of the Barkhausen effect.
The disadvantages of the known method include low sensitivity of the method and potential false indication of a free string in case of a high coercive force of the string metal, as well as in the necessity to take stationary measurements, which extends considerably the work period.
In the following known method of determination of the stuck point of drill pipes (Russian Inventor's Certificate 142242 E 21 B 23/09, 1961), discrete magnetic marks are successively created on the drill pipe, using a magnetizing coil. Then, the curve of magnetic induction (magnetic field intensity) along the pipe string is recorded, using a magnetic modulation sensor. A certain mechanical (twisting or stretching) force which is not to exceed the ultimate strength of the pipe is applied to the stuck pipe, and a magnetic induction curve is recorded again. Due to elastic deformation of the free part of the drill pipe, the magnetic marks demagnetize on this part of the pipe but remain on the stuck part, which is clearly observed on the magnetic induction curve.
The disadvantages of the known method include its complexity resulting from the necessity to perform the operation of creation of discrete magnetic marks, as well as insufficient accuracy resulting from the discrete pattern of arrangement of the marks.
Thus, known downhole conventional systems have limitations. In case of the anchored tool, the measurements are affected by the cable motion while applying stretch or torque from the surface. The rig-up methods for such measurements are often complicated and dangerous to personnel involved. The magnetic mark method precision is limited by the wireline depth control system resolution and so this method is often insufficiently precise.
The object of the present invention determination of the free point in stuck drill pipes is to increase the reliability and to simplify the procedure of determination of the free point in a string.
Another object of the method developed is to reduce costs of emergency maintenance works due to a reduced work period, as well as due to accurate determination of the stuck point.
The invention is based on the recognition that the magnetic permeability of a metal varies under tension.